1. Field of the Invention
This invention relates to an underbody structure of a vehicle, and more particularly to a reinforcement structure of a portion on which the rear end portion of a front frame and the front end portion of a side sill converge.
2. Description of the Prior Art
In view of the fact that the vehicle body supports the overall weight of the vehicle and determines the overall shape of the vehicle, the vehicle body must be provided with sufficient strength and rigidity, while on the other hand, from the viewpoint of fuel economy and running performance, it is preferred that the vehicle body be as light as possible. In order to satisfy these conflicting requirements, various vehicle body structures have been proposed. For example, it has been known to increase the rigidity of the vehicle body by providing on the front portion of the vehicle body a front frame having a closed cross section and extending in the longitudinal direction of the vehicle body along a front wheel apron, and providing in an intermediate portion of the vehicle body defining a passenger compartment a side sill extending in the longitudinal direction of the vehicle body along a side edge of a front panel.
However, since the front frame is usually positioned higher than the side sill and on the inner side of the same in order to avoid interference with the front wheel and the like, the front end of the side sill and the rear end of the front frame cannot be directly connected with each other, and accordingly, there arises a problem in the strength of the portion connecting the front end of the side sill and the rear end of the front frame. In order to overcome this problem, there is proposed in Japanese Unexamined Utility Model Publication No. 55(1980)-34904 a vehicle body structure in which the rear end of the front frame is connected to the front end of a floor frame extending in the longitudinal direction of the vehicle body along the lower surface of the floor panel on the inner side of the side edge of the floor panel, and the junction of the front frame and the floor frame is connected to the front end of the side sill by a torque box extending in the transverse direction of the vehicle body. Though being advantageous in that since the front frame, the side sill and the floor frame are connected by the torque box, the body rigidity is enhanced at the junctions of these members, the vehicle body structure gives rise to a problem in deformation of the vehicle body caused by rearward movement of the front frame upon collision against another vehicle. That is, since the front frame is positioned higher than the floor panel, the side sill and the floor frame, a moment which tends to rotate upward the front frame relative to the torque box to which the front ends of the side sill and the floor frame are connected is generated when the front of the vehicle body collides against another vehicle or the like and the front portion of the vehicle body is pushed rearward, thereby rearwardly moving the front frame. This moment rotates upward the front frame about the junction of the front frame and the torque box, thereby causing deformation of the upper portion of the vehicle body, which can cause deformation of the windshield mounting portion.